I'm sorry to ask, but what's the point in this?
Back when we discussed balance I told you that the way the game is built, NG+ can be entered at levels anywhere between 16 (linear play), 25-30 (all gold stars) and 40 (cap) - and unless you want to do scaling (which you don't), no matter what you do with the game difficulty, almost every player will have stages of "way too easy", "just fine" and "oh shit I need to head back and grind like mad".
Making the side quests in NG+ tougher means that you're basically telling any nongrinder "sorry, we don't cater for your style of play. Now go grind and return to those sidequests when your party is level 40+". Why?
I think that you should pick a target audience and tailor the difficulty for them - do not try to please everyone as it will backfire. It seems obvious to me that level 25-30 after beating Normal game is going to be the most-common one, as more or less everybody will want to get their hands on the powerful uniques plus bonus rewards before delving into NG+. So make the game good for them. If somebody beats the game, then spends 10+ levels grinding and then complains that NG+ is too easy for them, they have themselves to blame after all. (And if somebody tries to make a run through the game, as much as I like that play style, I can accept that we asked to have it tough.